Recently, a photocatalytic thin body using titanium dioxide has drawn attention. “Photocatalyst” is a substance having semiconductive physical properties, excited when light with energy higher than the band gap energy between the conduction and valence electron bands is irradiated, and thereby producing electron-hole pairs.
Titanium dioxide with an anatase type crystal structure is photo-excited by irradiation of light with wavelengths of 387 nm or shorter. This causes decomposition reaction based on redox reaction, and simultaneously, hydrophilization reaction different from the decomposition reaction (activity). So far, titanium oxide, tin oxide and zinc oxide have been known as a metal oxide simultaneously causing these two reactions. Strontium titanate and ferric oxide have been known as a metal oxide causing only the decomposition reaction, and tungsten trioxide has been known as a metal oxide causing only the hydrophilization reaction.
The above-mentioned reactions can be utilized to provide self-washing, deodorization, anti-bacterial and other functions, and a variety of members and groups of products coated with such a photocatalyst have been proposed.
Various methods for producing such a photocatalyst have been proposed including a binder method, a sol-gel method, and a vacuum evaporation method.
The binder method involves dispersing fine grains of titanium oxide in an adhesive binder, applying the resulting dispersion to a predetermined substrate, and heating and drying the dispersion. However, the method has a problem that since the fine grains of titanium oxide are buried in the binder, the photocatalysis tends to be deteriorated.
The sol-gel method is a method of obtaining a photocatalytic film by applying a liquid-phase agent including titanium chelate or titanium alkoxide containing titanium to a predetermined substrate, drying and then firing the agent at a high temperature of 500° C. or higher. However, the method has a problem that it requires the firing step at a high temperature of 500° C. or higher, which imposes considerable limitations on available substrates in terms of heat resistance.
Another problem in the so-called “wet methods” including the binder and sol-gel methods is the poor durability of the photocatalytic film. Furthermore, these wet methods require a film thickness of 400 to 600 nm for enhancing film crystallinity and obtaining a good photocatalysis. This film thickness also causes a problem that it is likely to create “color mottling” due to the interference effect in the visible light band.
In contrast to these formation methods, formation methods using the so-called “dry methods” such as a vacuum evaporation method and sputtering method have been proposed.
For example, Japanese Patent No. 2,901,550 discloses a photocatalyst having a layered structure of titanium oxide and silicon oxide formed by a vacuum evaporation method.
Also, Japanese Patent Application Laid-Open No. 2000-126613 discloses a method of depositing silicon oxide by a reactive sputtering method.
However, even in these dry methods, the relationship between the film formation condition and the performance of the resulting photocatalytic films is poorly understood and leaves room for improvement. Furthermore, these methods are to be improved in terms of productivity and cost, because the film formation rate in these methods is generally low.
The invention is accomplished based on such findings. An object of the invention is to provide a method of producing a photocatalyst having an excellent photocatalytic property, improved productivity, and a good dark place retention property, and an apparatus for producing a photocatalyst suitable for use in carrying out such a production method.